Numerical Investigations of Corner Singularities in Cracked Bodies

Abstract:

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The influence of corner singularity on the distribution of the stresses around the crack
front for a three-dimensional structure is described in this paper. The distribution of stress
singularity through the thickness of the middle tension specimen provides us an indication of crack
behaviour close to a free surface. An estimation of the region where the change of singularity
exponent plays an important role in fatigue crack propagation rate was carried out. A decrease in
fatigue crack propagation rate close to the free surface was found. The lower fatigue crack
propagation rate in the boundary layer in comparison with the interior of the specimen leads to a
change of intersecting angle between crack and free surface and depends on Poisson’s ratio.

Abstract: A two-parameter constraint-based fracture mechanics approach is used to explain the effect of the constraint on the apparently anomalous behavior of short fatigue cracks. The different levels of stress constraint are quantified by the T-stress, and microstructurally as well as mechanically short cracks are discussed. Short cracks generally behave more sensitively to the constraint than the long ones. It is shown that in most cases, the existence of short cracks goes hand in hand with an intrinsic loss of the constraint, which contributes to a decrease of their fatigue
threshold values and accelerates their growth. In this paper, the above effect is quantified and conclusions concerning the applicability of the fracture mechanics parameters and approaches to the estimation of the residual fatigue life of structures are discussed.

Abstract: In this study the three – dimensional surface cracking of a graded coating bonded to a homogeneous substrate is considered. The main objective is to model the subcritical crack growth process in the coated medium under a cyclic mechanical or thermal loading. Because of symmetry, along the crack front conditions of mode I fracture and plane strain deformations are assumed to be satisfied. Thus, at a given location on the crack front the crack propagation rate would be a function of the mode I stress intensity factor. A three – dimensional finite element technique for
nonhomogeneous elastic solids is used to solve the problem and the displacement correlation technique is used to calculate the stress intensity factor.

Abstract: The formulations of fatigue crack growth prediction are still mostly based on
phenomenological models. A commonly used formula in the field of high cycle fatigue is the Paris-
Erdogan law. For given experimental conditions (such as temperature, stress ratio or environmental
conditions) the parameters C and m have to be experimentally determined and considered as
material constants. Thus, for a given material, the fatigue crack growth rate (FCGR) depends only
on the applied range of the stress intensity factor. In a threshold region a significant shift in the data
of the fatigue crack propagation rate can be observed. The shift is induced by different test specimen
geometry. To analyses it the authors will present their own laboratory fatigue crack growth rate test
data measured on two different specimens with different levels of constraint and for different steels.
It is demonstrated that fatigue characteristics (i.e. C, m and Kth) obtained from different specimen
geometries are not only properties of the materials but depends on the specimen geometry.

Abstract: In this paper, the mechanisms of propagation of the damage in aluminum panels repaired with bonded composite patches of different mechanical characteristics is analyzed. The aim of this study is to analyze analytically, experimentally and numerically the advantage of the use of bonded composite patches to increase the fatigue life and to reduce the state of tension at the crack tips. The experimental results show that both static strength and fatigue life of the repaired aluminum panel has significantly increased due to the bonded composite patches. The different patches and adhesive, used for cracked panels, have provided about a 100-110% improvement in the fatigue life and a 30-35% decrease in the stress intensity factor. A comparison between finite elements calculations and experimental data has been carried out. The good agreement between the experimental data and the numerical ones has demonstrated the possibility to obtain an optimized design of bonded patches with the numerical tools.

Abstract: Based on the theoretical study on the tip stress intensity factor (SIF) of the crack normal to and dwelling on the interface, using the finite element software ANSYS, the SIFs of the double interface cracks normal to and dwelling on the interface in cladding material structure are studied by changing the crack spacing, the crack length, the cladding thickness ratio, the load and the crack location. The results show that, the crack SIFs become larger with the increase of the crack spacing, the crack length and the load, they become smaller with the increase of the coating thickness ratio, that the SIF of the crack close to the boundary becomes smaller with the increase of the distance between the crack and the boundary, and that the SIF of the crack in the middle of the interface becomes larger with the decrease of the crack distance.